ISL41334 INTERSIL [Intersil Corporation], ISL41334 Datasheet - Page 19

• Manufacturer Part Number: ISL41334
• Manufacturer: INTERSIL [Intersil Corporation]

The V
shown in Figures 20 and 21. All of the DC V
to inputs with internal pull-up resistors (DE, SP, LB, ON/OFF)
being driven to the low input state. The worst case I
occurs during SHDN (see Figure 20), due to the I
the ON/OFF pin pull-up resistor when that pin is driven low.
I
Figure 20 drops by about 40µA (at V
SP inputs are high (middle vs. top curve). I
RS-232 mode, because only the ON/OFF pin should be
driven low. When all these inputs are driven high, I
<1µA, so to minimize power dissipation drive these inputs
high when unneeded (e.g., SP inputs aren’t used in RS-232
mode, so drive them high).
Active Low Rx Enable (RXEN)
In many RS-485 applications, especially half duplex
configurations, users like to accomplish “echo cancellation”
by disabling the corresponding receiver while its driver is
transmitting data. This function is available on the QFN
package via an active low RXEN pin for each port. The
active low function also simplifies direction control, by
allowing a single Tx/Rx direction control line. If an active high
RXEN were used, either two valuable I/O pins would be
used for direction control, or an external inverter is required
between DE and RXEN. Figure 13 details the advantage of
using the RXEN pin.
IL
Tx/Rx
through an input pull-up resistor is ~20µA, so the I
FIGURE 13. USING ACTIVE LOW vs ACTIVE HIGH RX
L
ACTIVE LOW RX ENABLE
ACTIVE HIGH RX ENABLE
supply current (I
Tx/Rx
ENABLE
RXEN *
* QFN ONLY
DE
RA
DY
RXEN
DEN
RA
DY
ISL41334
L
) is typically less than 100µA, as
ISL81387
D
R
D
19
R
GND
+5V
V
B
A
Y
CC
Z
GND
+5V
V
B
A
Y
Z
CC
L
= 5V) when the two
L
L
is lowest in the
+
current is due
0.1µF
+
0.1µF
L
L
ISL81334, ISL41334
through
L
drops to
current
L
in
RS-485 Slew Rate Limited Data Rates
The SOIC and SSOP versions of this IC operate with Tx
output transitions optimized for a 20Mbps data rate. These
fast edges may increase EMI and reflection issues, even
though fast transitions aren’t required at the lower data rates
used by many applications. The ISL41334 (QFN version)
solves this problem by offering two additional, slew rate
limited, data rates that are optimized for speeds of 115kbps,
and 460kbps.The slew limited edges permit longer
unterminated networks, or longer stubs off terminated
busses, and help minimize EMI and reflections.
Nevertheless, for the best jitter performance when driving
long cables, the faster speed options may be preferable,
even at lower data rates. The faster output transitions deliver
less variability (jitter) when loaded with the large capacitance
associated with long cables. Figures 43, 44, and 45 detail
the jitter performance of the three speed options while
driving three different cable lengths. The figures show that
under all conditions the faster the edge rate, the better the
jitter performance. Of course, faster transitions require more
attention to ensuring short stub lengths, and quality
terminations, so there are trade-offs to be made. Assuming a
jitter budget of 10%, it is likely better to go with the slow
speed option for data rates of 115kbps or less, to minimize
fast edge effects. Likewise, the medium speed option is a
good choice for data rates between 115kbps and 460kbps.
For higher data rates, or when the absolute best jitter is
required, use the high speed option.
Speed selection is via the SPA and SPB pins (see Table 3),
and the selection pertains to each port programmed for
RS-485 mode.
Evaluation Board
An evaluation board, part number ISL41334EVAL1, is
available to assist in assessing the dual protocol IC’s
performance. The evaluation board contains a QFN
packaged device, but because the same die is used in all
packages, the board is also useful for evaluating the
functionality of the other versions. The board’s design allows
for evaluation of all standard features, plus the QFN specific
features. Refer to the eval board application note for details,
and contact your sales rep for ordering information.
December 20, 2005
FN6202.1